GB2187837A - Beam splitters - Google Patents

Abstract

In conventional beam splitters, some of the light passing from the input down each limb of the beam splitter will be reflected from the interface at the end of each limb to pass back to the input, thus forming a form of Michelson interferometer and generating spurious interference fringes. To avoid this, the optical path lengths from the point at which the light is split, to the output optical interfaces are caused to be sufficiently different to avoid the interference effect. When used in a fibre-optic interferometric sensor such as a laser gyroscope, the optical path length difference D should be at least the coherence length of the light source. <IMAGE>

Description

SPECIFICATION Beam splitters This invention relates to beam splitters for use in fibre optic interferometers and in particular, but not exclusively, to beam splitters for use in fibre optic gyroscopes.

In a fibre optic gyroscope, rotation is measured by sensing the relative phase shift introduced between two counter propagating beams of light by virtue of the Sagnac effect.

The phase shift may be sensed by determining the magnitude of a compensatory phase shift which is sufficientto null the phase shift induced by rotation. In a known form of gyroscope light from a light source is supplieid to a beam splitter or integrated optics Y branch to be split into two components which are subsequently propagated in opposed directions around a multi-turn coil of optical fibre.

Prior to entering the coil at least one of the component beams is passed through an optical modulator (e.g. a frequency or phase modulator) to enable the compensatory non-reciprocal phase shift referred to above to be applied. Two forms of architecture have been proposed; in one form a beam is supplied to an integrated optics Y branch waveguide having a single input and two output branches and having electrodes associated with at least one of the branches for applying a predetermined phase shift. In another proposal the input light is split by conventional means and then each component is supplied by twin lengths of optical fibre to an integrated optics device comprising twin parallel wave guides with associated electrodes.In each case, problems arise due to reflections from the integrated optics/optical fibre interface since these reflections are recombined by the beam splitter to form unwanted interference fringes which give rise to bias and drift in the output of the fibre optic interferometer.

It has been proposed to reduce the scale of the problem by reducing the amount of light reflected at the interface, for example by forming the interface so that it is at an inclined angle to the waveguide axis thereby to reduce the amount of light that is reflected back to the beam splitter. Another proposal is to reduce the reflections by matching the refractive indices of the integrated optics wave guide and the optical fibre. This however is a costly process and furthermore only reduces the scale of the problem rather than solving it.

According to one aspect of this invention there is provided a beam splitter for use in a fibreoptic interferometric sensor comprising an input for radiation, beam splitting means for splitting input radiation into two component beams, first and second waveguide means defining first and second paths of different lengths for said component beams and for be ing coupled to further waveguide means of different refractive index, said first and second path lengths being selected having regard to the coherence length of the input radiation such that the path difference is sufficient to reduce the visibility of interference fringes formed by the recombination of radiation reflected back along said first and second wave guide means.

By this arrangement, any radiation reflected back from the interface between the first and second wave guide means and the further wave guide is cut back to the required incoherency thereby leading to a reduction in visibility of the fringes.

In one arrangement, the beam splitter is formed as an intergrated optics device on a substrate of suitable material and optical modulating means are associated with at least one of the wave-guide means for modulating radiation passed thereby. In this arrangement the required path difference is achieved by slicing off a portion of the substrate thereby to reduce the length of one of said waveguide means relative to the other.

In another arrangement, the beam splitter may be constructed primarily of optical fibre, having fibres defining first and second waveguide means for supplying the first and second component beams to respective integrated optics or electro optics modulators. In this case, the path difference is achieved by adjustment of the length of one of the branches of optical fibre downstream of the beam splitter.

Further aspects will become apparent from the following description which is by way of example only and in which reference will be made to the accompanying drawing, in which, Figure 1 is a schematic diagram illustrating a fibre optic gyroscope; Figure 2 is a plan view of a first form of integrated optics beam splitter and modulator; Figure 3 is a plan view of a second form of integrated optics beam splitter and modulator; Figure 4 is a plan view of a third form of integrated optics beam splitter and modulator; Figure 5 is a plan view of a fourth form of integrated optics beam splitter and modulator, and Figure 6 is a view of an alternative view of beam splitter and modulator.

Referring initially to Fig. 1, the fibre optic gyroscope comprises a broad band light source 10 which supplies light via a Y branch 11 to a polarizer 12. The polarized light is then supplied to a further Y branch 13 to split it into two components (clockwise and counter clockwise) which, after modulation by modulators 14 and 15 propagate around a multiturn coil 16 of optical fibre. After leaving the coil, the clockwise and counter clockwise components are recombined by the Y branch 13, passed via polarizer 12 and Y branch 11 to a detector 17. A microprocessor (not shown) controls the various constituent parts of the gyroscope and processes the output of the detector in a known manner and outputs data representing the rate of rotation applied to the coil. These techniques do not however form part of this invention and will not therefore be described.

The Y-branch 13 and modulators 14 and 15 may be configured as an integrated optics device, as shown in Fig. 2. Here the Y-branch is formed by diffusing Titanium into a substrate of Lithium niobate 20 to form a Y shaped waveguide 21. Modulation of the beams is effected by means of electrodes 22, 23 deposited over each of the branches of the Y which apply an electric field across the waveguide thus to impart a phase shift to light passing through the waveguide. The free ends of the branches are coupled to the ends of the fibre optic coil 24.The refractive index of the waveguides is typically of the order of 2.1 whereas the refractive index of the optical fibre is typically of the order of 1.5 and thus a proportion of the light in the branches of the Y is reflected at the integrated optics/fibre optic interfaces 25, 26 in each thus forming a Michelson interferometer which gives rise to a drift and bias in the output of the gyroscope.

In order to avoid the problems so caused, the architecture of the gyroscope is adjusted so that the path lengths between the points in which the light is split into components and each of the interfaces 25, 26 between the integrated optics/optical fibre are different.

This may be achieved in several ways for example as shown in Fig. 3. In Fig. 3 an integrated optics Y branch waveguide 21 is formed on a substrate 20 and a modulator 22, 23 is associated with each branch of the wire. The substrate is cut to shorten the length of one of the branches by a distance ''D'' By shortening the lower branch as viewed in Fig. 3 by distance "D" the path length difference between the path followed by the component in the upper branch of the device from point ''S'' to the interface 25 between the integrated optics and the optical fibre 24 back to point "S" and the path followed by the component in the lower branch of the device from point ''S'' to the interface 26 and back to point S is twice the distance The amount by which the branch is shortened is selected to ensure that there is minimal coherence between the reflections of each of the components at point "S", thereby to reduce the visibility of any interference fringe.

The actual length necessary will depend on the coherent length of the substrate (i.e. distances "D") of the operating radiation, but a typical reduction of length will be of the order of 200 microns for a broad band light source.

Figs. 4 and 5 show further examples of integrated optics device in which the distances from the point "S" at which the light is split and each of thee interfaces 25, 26 are different.

Fig. 6 shows a form of arrangement in which the light is split by means of a fibre optic coupler or similar form of beam splitting arrangement. In this arrangement, the light passes from the polarizer via an optical fibre to the beam splitting coupling arrangement 30 into two further optical fibres 31, 32. In this arrangement, the modulators 22, 23 are formed as equal length parallel waveguides in an integrated optics device. In order however to prevent coherent, reflection at the optical fibre/integrated optics interfaces 25, 26 respectively the optical fibres differ in length by a distance "D" which as before is selected to minimise the coherence function.

In the above described arrangements, mention has been made of the use of a broad band radiation source. It would however be possible to employ a semi-conductor laser as a radiation source which produces light of a fairly narrow band. In this case, the path difference between the point at which the light is split and each of the optical fibre/integrated optics interfaces would be selected so as to minimise the coherence function.

In addition, whilst mention is made of integrated optics modulators, the beam splitting device could be used with any other optic system in which it is necessary to reduce bias and drift caused by reflections at the interface between two materials of different refractive indices.

In addition to imparting a path difference sufficient to reducing the coherence function of the reflections, the intensity of the reflections themselves may be reduced by angling the interface or by matcing of the refractive indices of the materials to either side of the interface.

Claims (6)

1. A beam splitter for use in a fibre-optic interferometric sensor comprising an input for radiation, beam splitting means for splitting input radiation into two component beams, first and second waveguide means defining first and second paths of different lengths for said component beams and for being coupled to further waveguide means of different refractive index, said first and second path lengths being selected having regard to the coherence length of the input radiation such that the path difference is sufficient to reduce the visibility of interference fringes formed by the recombination of radiation reflected back along said first and second wave guide means.

2. Interferometric sensor apparatus including: an optical radiation source; and an integrated optical waveguide device comprising a substrate supporting an integrated waveguide having a first waveguide portion which extends between an optical radiation entry portion of the substrate and an optical junction formed in the substrate, and second and third waveguide portions which each extend from said junction to two optical radiation outlet surface portions of the substrate, the device being operable for receiving radiation from said source into said first waveguide portion and for splitting the radiation at said junction to produce two beams which pass along respective ones of said second and third waveguide portions and exit from the device at respective ones of said two outlet surface portions;; said second and third waveguide portions having respective different lengths which differ one from another by at least the coherence length of the radiation produced by said source.

3. Apparatus according to claim 2, wherein said device further comprises electro-optical means coupled to said substrate for modulating the phase of the beam in at least one of the second and third waveguide portions.

4. Apparatus according to claim 2, wherein said entry surface portion is at one side of said substrate the two outlet surface portions are spaced apart at the opposite side of the substrate, and said opposite side is stepped, between the outlet surface portions, so that one of the outlet surface portions is nearer said one side of the substrate than the other outlet surface portion.

5. Apparatus according to claim 2, wherein said second and third waveguide portions have respective different shapes.

6. Interferometric sensor apparatus including: an optical radiation source; beam splitter means for receiving radiation from the source and splitting the radiation to form two component beams; two fibre-optic radiation conductor portions connected to the beam splitter means for receiving respective ones of said two component beams; and an integrated optical waveguide device comprising a substrate supporting two integrated waveguide portions, the device being coupled to said two fibre-optic radiation conductor portions for said two component beams to be received by said waveguide portions; said two fibre-optic radiation conductor portions having respective different lengths which differ by at least the coherence length of the radiation from said source.